MEMS structures are mostly dominated by one dimensional structure like beams and two dimensional structures like plates. Measurement of deflection of such elastic structures involves an important element in calibration and testing of MEMS devices. MEMS transducers mostly use piezoelectric, piezoresistive or capacitive as the transduction principles. As piezoresistive sensors are stress sensitive, their performances are closely related to the packaging technologies. Any mechanical stress or thermal mismatch between the sensor chip and the packaging material may cause a large offset voltage and a temperature drift of the output signal. Further, piezoresistance is susceptible to junction leakage and surface contamination. These factors may cause serious stability problems. A disadvantage of piezoelectric sensors is the relatively high noise level. Due to these problems, efforts have been made in the development of micromechanical sensors using capacitive sensing.
An important performance parameter for an elastic deflection sensor is higher overall sensitivity which is dependent on both the mechanical and the electrical sensitivity. The present day deflection measurement principles used in devices like microphones, pressure or any elastic deflection measurement sensors, produce a linear change in the output signal (voltage change) with a variation in the deflection of the elastic member. The present state-of-the-art is mostly based on designing the capacitance detection circuitry so as to be able to pick-up changes as low as femtofarads from electrical point of view as well as making the elastic member sensitive from mechanical point of view.
The design of elastic deformation sensors as applicable to MEMS microphones and pressure sensors has evolved over last 15 years with an aim of increasing sensitivity. The following are some of the design alternatives implemented till date so as to increase the sensitivity of the said sensors.                In MEMS microphones, the air trapped in between the diaphragm and the back-plate offers resistance to the movement of the diaphragm thus damping the response of the device which leads to a decrease in its sensitivity. As a solution to this, in [1] it has been suggested to decrease the damping due to air by increasing the acoustic hole density in the back-plate.        The inherent stress in the elastic member due to the deposition process makes it stiffer leading again to reduced sensitivity. The initial stress is regulated by controlling the deposition parameters or by choosing suitable material of the elastic member. An alternative method of increasing its sensitivity as suggested in [2] is to use Polysilicon as the material whose initial stress could be adjusted over a wide range.        The application of corrugated diaphragms in microphones or capacitive pressure sensors offers the possibility to control the mechanical sensitivity of the diaphragm by means of the dimensions of the corrugations, which are often easier to control than the parameters of a deposition process. A change in the design of the diaphragm by making shallow corrugations on its periphery instead of it being flat was another solution suggested in [3]. Compared to the microphones with conventional flat diaphragms, the microphone with a shallow corrugated diaphragm demonstrated improved sensitivity, especially for a high residual stress level. Similarly the non-planar diaphragms presented in [9] showed higher deflection compared to flat ones.        Another variation in the diaphragm design was suggested in [4] where the circumferential corrugation rings on the periphery of the diaphragm were replaced by a single deep corrugation. The distinct feature of the design is that the whole sensing area of the diaphragm is at the bottom of the corrugation. Thus, there would be no increase in its bending stiffness, which occurs in shallow corrugated diaphragms [5].        A highly sensitive silicon condenser microphone with integrated field-effect transistor has also been designed. Here the microphone membrane was suspended at four bending beams only and otherwise free to oscillate when exposed to sound pressure. The device showed a high sensitivity of up to 38 mV/Pa in the frequency range below 10 kHz [6].        In [7], a silicon condenser microphone with integrated field effect transistor has also been designed where the diaphragm works as the gate of the transistor. The transistor was biased in the ON state and the drain current was the response of the device.        Employing higher bias voltages i.e., stronger electrical field can increase the open circuit sensitivity, even though it is not preferred in many low voltage applications of microphones [4].        Reducing the stress of the diaphragm by modification of its supports has also been implemented in [8]. Essentially a diaphragm anchoring scheme was used which made the diaphragm more free to deflect.        